The purpose of these studies is to develop and validate a method to preoperatively determine the physiological significance of coronary obstructive lesions in man. The decision to perform coronary bypass surgery depends upon a judgment concerning the physiological significance of a given narrowing in a coronary vessel detected by angiography. Also, the evaluation of many noninvasive approaches to the diagnosis of coronary artery disease (perfusion imaging, exercise isotope ventriculograms, ECG exercise tests, etc) depend upon a judgment concerning the physiological significance of a coronary obstruction detected by coronary arteriography. Recent studies at Iowa involving measurements of coronary reactive hyperemia with a dippler system at the time of cardiac surgery have shown that the usual clinical method of assessing the physiological significance of a coronary obstruction (visual interpretation of the coronary arteriogram) frequently leads to erroneous judgments regarding the physiological significance of the coronary lesion. We plan to evaluate the potential usefulness of two approaches to improving the preoperative evaluation of coronary obstructive lesions: 1) A quantitative computer-based analysis of the coronary arteriogram: With this approach, we will assess minimal cross-sectional area of the lesion, percent area stenosis, and predicted stenosis resistance. Additional variables to be considered include estimates of the expected normal vessel size based on coronary branching pattern, and the patient's age, sex, and left ventricular mass. 2) Conventional noninvasive imaging procedures: rest and exercise isotope ventriculograms and thallium-201 perfusion scintigrams: Each of these approaches will be evaluated by comparing the results obtained with direct measurements of coronary reactive hyperemia, a physiological index of coronary reserve in humans, performed at the time of open-heart surgery. At the time of coronary occlusion, the presence of myocardial ischemia will be assessed by measuring regional wall thickening in the vascular field of the occluded vessel. Initial studies will focus on patients with proximal obstructions of the left anterior descending coronary artery.